Is the Errors & try/except lesson free?

Yes. The first 5 lessons on PyRun are free forever — no signup, no card. Errors & try/except runs real Python in your browser via Pyodide.

How long does the Errors & try/except lesson take?

About 8 minutes end-to-end. It's a hands-on lesson — you read a paragraph, edit the code, and run it in the same tab.

Do I need to install Python to try the Errors & try/except lesson?

No install required. PyRun runs real CPython in your browser via Pyodide (Python compiled to WebAssembly). Works on Chrome, Firefox, Safari, and Edge — desktop or mobile.

What level is the Errors & try/except lesson?

Intermediate level. Best after finishing the Beginner track (syntax, control flow, functions, data types).

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Intermediate·8 min·intermediate · errors

Errors & try/except

You’ll be able to

Catch specific exceptions with try / except blocks
Raise your own exceptions with raise
Use finally for cleanup that must always run
Read Python tracebacks to diagnose bugs faster

Why this matters

Bare except: clauses hide KeyboardInterrupt and SystemExit — a classic reason CI jobs won't die on Ctrl-C. Sentry, structlog, and every FastAPI exception handler depend on precise exception hierarchies and raise ... from err chaining.

Common pitfalls

except Exception: swallowing errors without logging — always log or re-raise; silent failures rot codebases.
Losing traceback context by re-raising as raise NewError(str(e)) — use raise NewError(...) from e instead.
Putting cleanup in except instead of finally — cleanup won't run on the happy path.

Python raises exceptions when something goes wrong. You can catch them.

The pattern

try:
    risky()
except SomeError as e:
    handle(e)
finally:
    cleanup()

Best practices

Catch the narrowest exception you can. Bare except: hides real bugs.
Don't catch what you can't handle. Letting an error bubble up is often the right call.
Use finally: for cleanup that must run either way.

Try it

Add a check for negative b and raise a ValueError.
Wrap the calls in a try and print every error you get.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Write a function safe_int(s) that returns int(s), or returns 0 if the conversion raises ValueError. Then print safe_int("42") and safe_int("abc"). Expected outputs: 42 then 0.
Exercise 2
Write checked_divide(a, b) that returns a / b, raises ValueError("cannot divide by zero") when b == 0, and lets other errors bubble up. Then wrap checked_divide(10, 0) in a try/except and print the caught message.
Exercise 3
Given pairs = [(10, 2), (5, 0), (8, 4), (7, "x")], loop through and print each division result, or the error message for any pair that fails. Expected output includes 5.0, an error for the zero, 2.0, and a TypeError message.

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Errors & try/except

You’ll be able to

Catch specific exceptions with try / except blocks
Raise your own exceptions with raise
Use finally for cleanup that must always run
Read Python tracebacks to diagnose bugs faster

Why this matters

Common pitfalls

except Exception: swallowing errors without logging — always log or re-raise; silent failures rot codebases.
Losing traceback context by re-raising as raise NewError(str(e)) — use raise NewError(...) from e instead.
Putting cleanup in except instead of finally — cleanup won't run on the happy path.

Python raises exceptions when something goes wrong. You can catch them.

The pattern

try:
    risky()
except SomeError as e:
    handle(e)
finally:
    cleanup()

Best practices

Catch the narrowest exception you can. Bare except: hides real bugs.
Don't catch what you can't handle. Letting an error bubble up is often the right call.
Use finally: for cleanup that must run either way.

Try it

Add a check for negative b and raise a ValueError.
Wrap the calls in a try and print every error you get.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Write a function safe_int(s) that returns int(s), or returns 0 if the conversion raises ValueError. Then print safe_int("42") and safe_int("abc"). Expected outputs: 42 then 0.
Exercise 2
Write checked_divide(a, b) that returns a / b, raises ValueError("cannot divide by zero") when b == 0, and lets other errors bubble up. Then wrap checked_divide(10, 0) in a try/except and print the caught message.
Exercise 3
Given pairs = [(10, 2), (5, 0), (8, 4), (7, "x")], loop through and print each division result, or the error message for any pair that fails. Expected output includes 5.0, an error for the zero, 2.0, and a TypeError message.